I just started last year and have been well-served with a basic Createx transparent pack that included white and black. I bought the opaque pack at the same time but don't use them very often. A big bottle of white and black are good advice, as stated above, as they run out fastest. I don't run production, so having a whole bunch of pre-mixed stuff wasn't important. I can pretty much get any color I'm looking for by mixing. I did recently purchase some gray, since I tend to mix that a lot, and some flourescent orange. The flourescents are harder to get mixed and look good, in my experience.
I almost never spray a color straight out of the bottle, usually blending it with something else to get what I want. I haven't done much with the pearlescent colors. I couldn't get them to work like I wanted, so I don't think about them any more. I did have some luck with a very light coat of spray glitter I swiped from the wife's crafting box.
I can see how either mixing you own in larger batches and saving it or having a wider range of pre-mixed colors would be essential if you were doing a whole bunch of the same pattern and color. At most I do about 5. One for me and a few for friends. If you're going to use my "mix as you go" method I'd suggest doing them all in one sitting if you want them to be the same. Even with a formula I have problems getting exact matches consistently by using the "1 drop of this, 4 drops of that" method. You can get close, of course. I expect this is because not all drops are created equal.
Good luck painting, it sure is fun.
-SB